We are developing models of leukemia in mice using various murine leukemia viruses (MuLV). These viruses differ in their tropism for specific subclasses of B and T cells. Two different patterns are observed: (a) A given MuLV replicates in one cell class but not in another one; and (b) Two strains of MuLV replicate in the same lymphocyte class but transform different cells. Thus, there must be specific host cell factors regulating both permissive infection and transformation. Our goal is elucidation of these cell factors.